Moilanen arc without snowguns

I was recently filing images digitized from somewhat older slides and came across this upper tangent arc image from many years ago, in the early morning from here in Calgary. I decided to subject this image to some processing, in particular, embossing the image in Photoshop and low and behold, a faint Moilanen arc showed up, as you see in this composite image!

I certainly do not live anywhere near to a ski slope or factory chimney that would produce lots of ice crystals. There is no evidence on the photo of local ice crystals as we see often in diamond-dust displays. The climate here is very dry and we often find low-lying layers of diamond dust particularly over our river.

Thus, I hesitate to claim that this is a "natural" Moilanen arc but it was not taken near to ski slopes. Maybe our furnaces produce the requisite crystals sometimes, in which case I will be on the lookout next winter for a repeat of this display.

Alan Clark

Suggestion of observation: possible spurious arcs from Moilanen crystals

Here is an observation suggestion for next winter. Marko Mikkilä photographed possible new arcs during winter 2007-2008 ( 1 ). Source elevation was -1.5°. Different processing shows them as not only 22° arcs but also new helic arcs.

Some twinned crystals allow creating both similar arcs and Moilanen arc so these arcs could be spurious arcs from Moilanen crystals. Such twinned crystals are present in small amount in samples from Moilanen display. I have put the best images in the main picture. The crystals seem to be two twinned plates with a glue area in between them. Even few of these crystals could give significant Moilanen arc as the ray path is extremely efficient, even more efficient than parhelia ray path (for a plate crystal, rays go mostly in the helion, then in the left/right parhelia/subparhelia and parhelic/subparhelic circle while almost all the rays entering such twinned crystal go in the Moilanen arc (not in the helion)). The 22° arcs would be from rays entering and leaving the small upper tilted prism faces (with total internal reflections in between). The helic arc is mostly from total internal reflection on the small upper prism face.

Some reasons also tend to show that the actual fall orientation of such crystal is with the prism pointing up (and not pointing down like it was thought before).

If this is true, we can deduce that best configuration to see these arcs again is with source elevation around -10°. In this case, 22° arcs rays travel horizontally inside the crystal and so are more likely to exit (as the main habit of the crystals seems to be long needles), helic arcs should also be stronger, and tangent arc is narrow (so well separated from these arcs). However Moilanen arc should be difficult to distinguish from subsun and divergent light pillar.

For similar lower arcs, the best configuration would be source elevation around 10°. However lower arcs are less likely because of the “glue area” messing with the basal faces for total internal reflection.

It would be really interesting for people who have the opportunity to try to get other observation of these arcs next winter. They should be most visible at source elevation around -10° and vanish quickly for positive source elevation. If these arcs are spurious arcs from Moilanen crystals, it would definitely help finding the responsible crystals.

Nicolas Lefaudeux

Odd-radii again

Alan Clark sent the above image, which shows the same display that Elmar Schmidt saw from Virginia, and which can be read about in the previous post. Alan photographed the halo from Dulles Airport, about 120 km NE of Elmar's site. The Sun elevation was about 58 degrees, and it was between 15:00 to 16:00 local summer time. As Alan says: "The display was not perhaps as clear as Elmar's in that some components of the odd halo were not distinct." ( 1 )

Alan has sent another pair of odd radius halo images. These were taken from Calgary, Alberta, Canada in May, 2009. ( 2 - 3 )